Expose AADTokenCredential as part of Microsoft.Azure.Quantum

[anpaz]

Currently, the AzureSettings in the EntryPoint has a custom TokenCredential class (the AADTokenCredential that can be used if the user already has a acquired a Token.
It's not a big class, but it's convenient and might be useful in other scenarios where the AAD token is already available; however it is a private class in the EntryPoint package so it's not available to reuse. A better location would be to add it to the Microsoft.Azure.Quantum.Client package, with the rest of the Authentication helper classes.

Describe the solution you'd like
Move the AADTokenCredential from AzureSettings to Client.Authentication

Describe alternatives you've considered
Re-implement the AADTokenCredential when required.